A new high-pressure mass spectrometer with improved reaction temperature measurement

The high-pressure mass spectrometric technique (HPMS) can provide detailed gas-phase thermochemical data including free energies, enthalpies, and entropies for various ion-molecule reactions. Several problems related to accuracy of the HPMS method have been previously identified. These problems in addition to accuracy of ion source temperature measurement and control were considered in our attempt to construct an overall HPMS experiment that would lead to an improved accuracy in the derived thermochemical data. Design of the new ion source together with other construction details of our HPMS apparatus are described in this paper. The performance of the newly constructed high-pressure mass spectrometer was evaluated by studying the thermochemistry of stepwise hydration of chlorine anion in the gas-phase. Density functional theory calculations were performed to aid in choosing appropriate experimental conditions. The experimental and theoretical results exhibit non-monotonic trends in stepwise enthalpies and entropies of chloride ion hydration that are in disagreement with the results of previous HPMS studies. Detailed analysis of HPMS experimental errors is given in an attempt to explain the discrepancies.